This invention relates to centrifugal gas compressors and, more particularly, to an impeller and shaft assembly used in a high-speed gas compressor in a refrigeration plant or other chiller.
Centrifugal gas compressors have one or more impellers rotated in a cavity for compressing a gas, such as refrigerant vapor. The one or more impellers are mounted on a pinion shaft that is turned by a motor. In centrifugal gas compressors, it is important that the impellers and pinion shaft mounting arrangements are simple and efficient to manufacture, install and operate. In particular, overly complex attachment arrangements involving the machining of complementary grooves and threads in male and female parts pose a greater burden on highly skilled machinists, a resource that is both finite and costly. More particularly, such arrangements are more likely to be damaged during transport, installation and normal running of the compressor.
U.S. Pat. No. 4,257,744 describes an impeller and shaft assembly that includes a cap screw, a Belleville washer or spring, a deformable socket machined into the rear of an impeller, a drive shaft with a frusto-conical shaped extremity, and a steel washer. The impeller has an axial bore extending through its center and a counterbored recess at its front. The frusto-conical shaped extremity includes axially extended grooves that are circumferentially spaced and alternate with intervening lands. A high torque applied to the cap screw results in plastic deformation of the lining of the socket in the rear of the impeller.
The manufacture of the frusto-conical shaped extremity is complex and adds to the cost of the impeller and shaft assembly. In addition, the counterbored recess is sized to accommodate the cap screw. As a result, the protective steel washer and single spring are both sized to correspond to the cross-section area of the counterbored recess and screw cap. Thus, the torque results in a clamping force being directly transmitted from the cap screw, without dissipation, through the single Belleville washer and steel washer. This arrangement may damage the single Belleville washer and cause stress fractures in the front face of the impeller immediately around the counterbored recess, necessitating the costly replacement of the entire impeller. Thus, there is a need for a simple impeller and shaft assembly that minimizes the risk of damage to the front face of the impeller necessitating the costly replacement of the entire impeller.
Maintenance personnel may use an ordinary wrench when a torque wrench is more appropriate. Dramatic over or under-torquing of pinion shafts in centrifugal impeller configurations leads to increased maintenance and downtime costs. An impeller assembly that is less vulnerable to such problems is needed.
Additionally, cap screws increase the diameter of the impeller eye. The impeller eye is the terminal area on the cap screw end which is located radially inward of the impeller contour.
Other factors are the effect of thermal expansion of the aluminum impeller versus the steel drive shaft, and the fretting between the parts.